The search for oil and gas reserves over the past several decades has lead to the need for exploration in ever deeper waters. This in turn has lead to the need for offshore producers to build structures that can withstand strong ocean currents that could threaten the structural integrity of pipelines, risers or other immersed components.
The VIV oscillations of marine risers are known to increase drag, and have led to structural fatigue. One proven means of suppressing this vibration is the use of fairings and strakes. These coverings essentially modify the flow along the cylinder, tripping the production of Karman vortices so that they act less coherently or far enough downstream so they interact less with the body. In the existing prior art there are two general types of structures, helical strakes and fairings, for the suppression of VIV or vortex induced motions (VIM) around vertically disposed immersed objects such as risers or other supportive construction elements.
Helical Strakes:
Helical strakes are attached on the outside of a structure in order to suppress VIV by altering the vortex shedding pattern as well as the correlation of vortices shed along the length of the specimen. The most common helical strake geometry is the three-start strake. This consist of three triangular or trapezoid profiles which are helically wound and extend along the length of the specimen. The profiles can be permanently fixed to the specimen, or more commonly; attached using modules that are attached to the specimen. Regardless of attachment method, helical strakes are not designed to shift any during operation but rather stay in a stationary position relative to the object. Two main parameters defines the global shape of strakes: pitch (P/D) and strake height ratio (h/D), where P designates the pitch of the strake in relation to the main direction of flow, D is the outside diameter of the cylinder and h is the external cross-sectional distance from the cylindrical member to the strake-tip. In addition, the local geometry of the strake profile itself characterizes the helical strakes.
Fairings:
Fairings are attached to a structural member in order to alter the vortex shedding pattern of that member when subject to ambient fluid flow. Fairings are attached in a way that allows for the fairing to rotate around the center of the structural member (for example a marine drilling riser) to which it is attached. This allows for the fairing to align with the direction of the ambient flow. Various cross sectional designs of fairings exist today.
In addition, other means of suppressing VIV also exist, such as perforated shrouds, but all suffer from some negative aspects that favour the two groups of concepts above to be used widely in industry today.
Such prior art systems as mentioned above are documented in the literature and are given in the following. Books and papers on suppression on Vortex induced vibrations and methods of VIV suppression:    Sarpkaya, T., 1979, “Vortex-induced oscillations”, Journal of Applies Mechanics 46, pp. 241-258.    Blevins, R. D., 1990, Flow-induced Vibrations, Van Nostrand Reinhold: New York, USA.    Griffin, O. M. & Ranberg, S. E., 1982, “Some recent studies of vortex shedding with application to marine tubulars and risers”, ASME Journal of Energy resources Technology, 104, pp. 2-13.    Bearman, P. W., 1984, “Vortex shedding from oscillating bluff bodies”, Annual review of Fluid Mechanics, 16, pp. 195-222.    Zdravkovich, M. M., 1997, Flow around circular cylinders, Vol. 1: Fundamentals, Oxford University Press: London, UK.    Naudascher, E & Rockwell, D., 1993, Flow-Induces Vibrations: An Engineering Guide. Balkema: Rotterdam, Netherlands.    Faltinsen, O. M., 2005, Hydrodynamics of High-Speed Marine Vehicles. Cambridge University Press.    Kristiansen, T., 2009, Two-dimensional numerical and experimental studies of piston-mode resonance. Ph.D. thesis, Norwegian University of Science and Technology.    Newman, J. N., 1977, Marine Hydrodynamics. The MIT Press, Cambridge, Mass.    Sumer, B. M. & Fredøse J., 1997, Hydrodynamics around Cylindrical Structures. World Scientific: Singapore.    Skaugset, K. B., 2003, On the Suppression of Vortex Induced Vibrations of Circular Cylinders by Radial Water Jets, Ph.D. thesis, Norwegian University of Science and Technology.
Patent publication in this field includes the following: U.S. Pat. Nos. 5,410,979, 5,421,413, 5,984,584, 6,010,278, 6,067,922, 6,179,524B1, 6,196,768B1, 6,223,672B1, US2006/0021560A1 and EP2049805B1.
In the following, the status of the mentioned prior art is explained in more detail. In terms of helical strakes known from the prior art, the following aspects and limitations should be noted:
Ability to Suppress Vortex Induced Vibrations (VIV):
Specific dimensions of helical strakes are needed to achieve proper VIV suppression characteristics. The pitch and strake height are vital parameters. In general, increasing the strake height has a positive effect on VIV suppression characteristics. However, this comes with the price of high drag forces.
High Drag Forces:
As explained above, helical strakes will increase drag forces on the structural member. This represents a structural capacity issue as well as potential operational limitations. In the case of a marine drilling riser, this can limit the operation in terms of pre-tension, top and bottom angle limitations and maximum tension in the riser. As a result, the drilling unit may be forced to suspend drilling operations in strong currents.
In terms of fairings, and in particular marine fairings known from the prior art, the following aspects and limitations should be noted:
Ability to Suppress Vortex Induced Vibrations (VIV):
The main reason for attaching fairing as a VIV suppression device is to reduce vibrations and material fatigue on the structure. However, existing fairings have varying suppression performance. For example, the flow condition window in which prior art fairings work as intended is limited. As the flow condition experienced (such as ocean currents) is not deterministic but varies in magnitude and direction for a given location, it is vital to attain excellent VIV suppression characteristics for all operating conditions. Hence a fairing design that only works in a specific flow condition window is undesirable.
Global Stability:
Existing fairings may become globally instabile for specific current flow conditions. This is a resonance phenomenon. The motions associated with such global instability can be devastating for a structural member. Motions will become considerably greater than the ones associated with VIV response, and may cause rapid material fatigue or structural overload. For an offshore application, the associated potential for loss of containment of hydrocarbons and the Health, Safety and Environment (HSE) risk can be relatively high.
Global Loads on the Structure:
Fairings are in general associated with relatively low drag forces. However, there is much room for improvement compared to existing designs. Local forces on the fairing are known to harm fairings during deployment, retrieval and operation. This is especially the case for large fairings and associated with interactions from waves either in the moonpool or in the upper part of the water column where wave action is most predominant. Fluid forces on the individual parts of the fairing may become large enough to cause the fairing to structurally disintegrate or get stuck, preventing weather-vaning, during operation. As this is known to have halted drilling operations, there is a need for new fairings to be small and robust in order to avoid this.
Robustness
Operational Issues:
When deploying fairings on an offshore drilling unit several key operational challenges are associated with the fairing size and weight. Small, light fairings could overcome many operational issues.
Storage:
Present fairings or helical strakes require relatively large storage space on a drilling unit. Some drilling units may have very limited space to carry such devices.
Installation and Retrieval Time and Cost:
A key cost factor for a drilling operation is the time to deploy and retrieve a marine riser. Using traditional helical strakes or marine fairings will halt normal operations due to manual labour involved in attaching the VIV suppression devices onto each joint of the marine riser. Increased total deployment and retrieval time will not only increase total time spent on the operation, but also increase demand on the available weather window needed to perform the operation. Offshore drilling units charge high daily rig rates, hence increased time for installation and deployment can prove very costly.
Installation and Retrieval HSE:
High unit weight and size of a VIV suppression device is not only time consuming, but does also represents an HSE risk in the installation and retrieval phases.
Installation Feasibility:
Due to limited space on the drilling floor, a simpler and more compact system and method for storage and deployment is needed.